ACTS was an experimental NASA satellite that has played a central role in the development and flight-testing of technologies now being used on the latest generation of commercial communications satellites. The first all-digital communications satellite, ACTS supports standard fiber-optic data rates, operates in the K- and Ka-frequency bands, and has pioneered dynamic hopping spot beams and advanced onboard traffic switching and processing. The ACTS program was managed at NASA/GRC (Glenn Research Center).

Launch: ACTS was launched on Shuttle mission STS-51 on Sept.12, 1993 from KSC (Kennedy Space Center), FLA. After reaching its operational orbit, the ACTS spacecraft was deployed from the Shuttle cargo bay into GTO using the TOS (Transfer Orbit Stage) booster of OSC. Several hours later, an apogee kick motor circularized the satellite's orbit, placing ACTS into position to drift slowly toward its final orbital destination by Sept. 28, 1993 (GEO). 1)2)

Orbit: Geostationary orbit at location 100º W (during most of its operational service life).

Technology introduction: The ACTS communications payload encompassed the introduction of several key technologies which supported the full range of on-demand voice, video and data communications services. The following technologies were tested: 3)

• Kaband (30/20 GHz with 2.5 GHz bandwidth) operation

• Demonstration of onboard switching, and the use of “hopping” spot beams.

Note: A hopping spot beam is an antenna beam on the spacecraft that points at one location on the ground for a fraction of a millisecond (ms). It sends/receives voice or data information and then electronically “hops” to a second location, then a third, and so on. At the beginning of the second millisecond the beam again points at the first location. - Forerunners to the ACTS satellite have used broad coverage beams that provided relatively low signal levels, requiring large ground receiving antennas, to the point that the uplink could only be done from very specialized and expensive stations. Spot beams, by contrast, concentrate the energy into small beams with very small footprints, providing in the case of ACTS an improvement of 20 dB in signal level, resulting in small diameter antennas and higher throughput for each Earth station. The use of spot beams, however, necessitates a large number of stationary beams or a rather small number of hopping beam families to cover a large geographic area.

• The deactivation of ACTS occurred on April 28, 2004 when funding was terminated. At the time, the geosynchronous orbit had an inclination of 1.8º (increasing by 0.8º per year). The ACTS mission lasted 127 months (instead of the originally planned 24-48 months) and provided pioneering communications research during its lifetime. 4)5)6)

From a programmatic standpoint, ACTS was also a very odd bird. It was essentially a government-sponsored demonstrator for commercially usable communications technology. ACTS demonstrated Ka-band technology for communications satellites, and was the latest—and last—in a line of communications technology satellites built for NASA since the early days of the space age. 7)

• 24-hour availability of spot beams until February of 2001 - decreased slowly after that date (due to pointing limits). The steerable antenna remained fully available.

• Also in May 2000, NASA extended ACTS availability for education and research purposes to an education-based consortium in Ohio, referred to as OCACT (Ohio Consortium for Advanced Communications Technology). OCACT formally assumed control of ACTS, under the provisions of a Space Act Agreement between NASA/GRC, the Ohio Board of Regents, and Ohio University. 8)9)10)11)

• On May 31, 2000 the ACTS experiments program officially came to a close. Experiments were continuously supported for 78 months of operations.

• In April 2000 as ACTS neared its planned end of mission, the routine east/west station keeping maneuvers that maintained this 100° position were discontinued. ACTS began a slow natural drift towards the gravity well at 105.2°. On June 18, 2000, a 5-minute burn of the West facing hydrazine thrusters on the spacecraft accelerated the drift rate to 0.075°/ day. To move a spacecraft in orbit, the orbit height is either raised (moves West), or lowered (moves East) from the geostationary altitude. The ACTS spacecraft arrived at its new parking orbit location (105.2º W) on Aug. 11, 2000. 12)

• In July 1998 the spacecraft's north/south stationkeeping was discontinued (to conserve the last amount of fuel) and to extend operations in an inclined orbit. The reason: the hydrazine fuel was almost consumed by the ACTS spacecraft to maintain its desired position in space.

• In Nov. 1997 a record data rate of 520 Mbit/s TCP/IP (Transmission Control Protocol/Internet Protocol) was achieved using ATM via ACTS and several ground stations. The transmission experiment involved government agencies and commercial partners (the ACTS 118 consortium of researchers) to test their COTS products, all were interested in testing the ability of ATM technology to transfer data at high rates over satellite. ACTS communication capabilities continued to be used by industry, universities, and government to develop new satellite services, including real¿time TV transmission to airliners. 13)14)15)

• By 1994, ATM had established itself as the network infrastructure of the Internet Age. The investigations have helped to define and identify the extensions to the Internet protocol suite that are beneficial to delivering Internet content over network paths containing long-delay satellite channels. In addition, NASA was interested in employing off-the-shelf Internet protocols to meet its near-Earth communication needs.- Use of VSAT ground stations and data rates between roughly 0.75 and 1.5 Mbit/s (i.e., between half and full T1 rate)in all experiments. 16)

• After its launch in September 1993, and 2 ½ months on-orbit system checkout, the Experiments program began on December 1, 1993 (i.e., the spacecraft was declared 'operational').

Spacecraft:

ACTS was developed as an experimental on-orbit, advanced communications satellite test bed, bringing together industry, government, and academia to conduct a wide range of technology, propagation, and user application investigations. The ACTS spacecraft was built by Lockheed Martin (formerly RCA Astro Space of East Windsor, NJ) as prime contractor. It has a mass of 1480 kg at the beginning of its on-orbit life; and it measures 14.4 m from tip to tip of the solar arrays and 9.1 m across the main receiving and transmitting antenna reflectors, with a height of 4.6 m from the spacecraft separation plane to the tip of the highest antenna. The main receiving antenna is 2.2 m in diameter; the main transmit antenna is 3.3 m in diameter. ACTS also incorporates beacons at 20.2 and 27.5 GHz. An operations phase of 24 months was planned. 17)18)19)

The ACTS spacecraft is 3-axis stabilized using sun and Earth sensors and a momentum wheel as actuator. The pointing accuracy is 0.025º in pitch and roll, 0.15º in yaw using autotrack ( 0.1º pitch and roll, 0.25º yaw using Earth sensor). In addition, an offset pointing control capability of ±6º in pitch, and ± 2º in roll is provided.

The bus is a Lockheed Martin 4000 series standard platform. The spacecraft bus structure is a box of size: 2.0 m x 2.1 m x 1.9 m. The communication antenna assembly has a height of 2.95 m above the antenna panel and a deployed width of 9.1 m. The spacecraft uses passive and active thermal control with blankets and heaters. Electric power of 1400 W (EOL) is provided by by the solar arrays. An addition, there are 2 NiCd batteries each of 19 Ah capacity. The power bus uses a voltage of 35.5 V (± 0.5 V) with full solar array illumination. On-orbit propulsion is provided by a blowdown hydrazine system with redundant thrusters and four tanks; the propellant mass is 263 kg. There are 16 thrusters (0.9, 2.2, and 4.5 N). The station keeping is within ±0.05º. 20)21)

Spacecraft type

3-axis stabilized communication technology satellite

Application

Testbed of new technology applications available to US experimenters free of charge

The MBA is comprised of separate Ka-band receive and transmit antennas, each with horizontal and vertical polarization subreflectors. Antenna feed horns produce narrow spot beams with a nominal 200 km coverage diameter (0.3º beam width) on the surface of the Earth. Fast (< 1 µsec), beam-forming switch networks consisting of ferrite switches, power dividers and combiners, and conical multi-flare feed horns provide sequential hopping from one spot beam location to another. These hopping spot beams interconnect multiple users on a dynamic, traffic-demand basis. A separate 1 m, mechanically-steered antenna, receiving uplink and radiating downlink signals, is used to extend the ACTS communication coverage to any location within the hemispherical field of view from the ACTS location at 100º west longitude. Beacon signals at 20.2 GHz and 27.5 GHz are radiated from two small, separate antennas. 23)24)25)26)

The MBA provides dynamic coverage with fixed and hopping spot beams. Each hopping spot beam can be programmed to sequentially cover a set of spots and dwell long enough to communicate with users in each spot. By assigning each user an access time, several users can transmit and receive at the same frequency on a time-shared basis. This TDMA (Time Division Multiple Access) technique requires a switching system onboard the spacecraft to interconnect the beams and route messages. The ACTS communication payload provides two types of onboard switching to interconnect the multiple spot beams and route signals to their appropriate destinations: 1) base band processing (BBP) and 2) microwave switch matrix (MSM).

TheBBP is a high-speed digital processor on the satellite that provides on-demand, circuit switching for the efficient routing of traffic among small user terminals. In essence, the BBP is the first switchboard in the sky to perform the same functions done by terrestrial telecommunication switch centers. Because its network is completely interoperable with the terrestrial system, ACTS can be considered a single node in a combined satellite/terrestrial network. ACTS conducts both time and space switching on board the satellite. The BBP switches traffic between the various uplink and downlink beams, automatically accommodating on-demand circuit requests.

In the BBP mode of operation, four simultaneous and independent hopping beams (two uplink and two downlink) provide flexible, demand access communication between small (1.2 m diameter antenna dish) user terminals with a maximum throughput of 1.79 Mbit/s or 28 circuits each of 64 kbit/s. Each uplink spot beam receives multiple channels. A user terminal is assigned an uplink channel and transmits its information using TDMA (Time Division Multiple Access). At the spacecraft, the receive signals are demodulated, decoded as required, temporally stored in memory, routed on a 64 kbit/s individual circuit basis, modulated, encoded if required, and transmitted in the proper downlink spot beam using a single TDMA channel. During the 1 ms TDMA frame time, the beams hop to many locations, dwelling long enough to pick up or deliver the required traffic. 28)29)

MSM (Microwave Switch Matrix):

The MSM is an intermediate frequency (IF) switch capable of routing high volume point-to-point traffic and point-to-multipoint traffic over 900 MHz bandwidth channels. Using satellite-switched TDMA, the microwave switch matrix dynamically interconnects three uplink and three downlink beams. The user terminals transmit TDMA bursts according to their destination.

At the satellite, the 30 GHz bursts are down-converted to an intermediate frequency, routed to the proper downlink beam port, up-converted to 20 GHz, and transmitted on the downlink. The switch paths are changed during guard intervals between bursts. Fixing the beam interconnections in a static mode allows additional flexibility for a variety of continuous digital or analog communication. The MSM mode accommodates user terminals operating from a few kbit/s up to 622 Mbit/s.

The ACTS system can be configured in the BBP mode, the MSM mode, or a mixed mode. In the mixed mode, both the baseband processor and the microwave switch matrix are operated simultaneously with some restrictions. The system can be quickly reconfigured from one mode of operation to another in a matter of minutes, further adding to the system's flexibility. This flexibility, along with the large total information throughput capacity, allows a large variety of users to be accommodated concurrently.

Ground segment:

The ground segment is comprised of the spacecraft and communication network control stations and the user terminals.

• The master command and control transmit and receive facilities are located at NASA/GRC, Cleveland, Ohio. In addition, NASA/GRC provides network control for all user communications. As part of network control, it processes and sets up all traffic requests, assigning traffic channels on a demand basis.

• A SOC (Satellite Operations Center) was located at Lockheed Martin Astro Space in East Windsor, New Jersey (in June 1998, the SOC was transferred to the Lockheed Martin Communications and Power Center facility in Newton, PA). The SOC has the prime responsibility for generating spacecraft bus commands and for analyzing, processing, and displaying bus system telemetry data. Orbital maneuver planning and execution are also handled by the SOC.

Terminals operated by various private, governmental, and university organizations validated these services. In addition, more than ten receive-only propagation terminals were used for propagation studies and modeling. 30)31)32)

• The technology experiments have been concentrated, but not limited, to T1 (1.55 Mbit/s) VSAT system availability. These experiments analyzed the effects of Ka-band system variances and propagation on the ACTS T1 VSAT performance using statistical performance indexes such as system fade availability over several years.

• Rain fade compensation characterization experiment: This is a process whereby a VSAT data channel BER (Bit Error Rate) performance is automatically enhanced during a period of signal attenuation due to rain or thermal distortion on the spacecraft antenna. The rain fade compensation protocol provides 10 dB of margin by reducing burst rates by half and invoking 1/2 convolutional code, constraint length 5. The result is a reduction of the 100 Mbit/s burst rates to 55 Mbit/s and the 27.5 Mbit/s burst rates to 13.75 Mbit/s. The protocol is adaptive in that it includes a decision process so that fade compensation is implemented only when needed.

Rain attenuation is the dominant propagation impairment at Ka-band frequencies. It is a function of frequency, elevation angle, polarization angle, rain intensity, rain drop size distribution, and rain drop temperature. The communication link performance experiment included more than seven sites in North America and lasted for a period of five years.

• A HDR (High Data Rate) terminal equipped with a 3.4 m antenna was developed to utilize the ACTS 900 MHz transponder bandwidth. HDR terminals seamlessly operated with terrestrial fiberoptic systems connecting supercomputers using ATM (Asynchronous Transfer Mode) at rates up to 622 Mbit/s and have transported high definition video and point-to-point traffic at rates of up to 520 Mbit/s.

• The GSN (Gigabit Satellite Network) was a system of five HDR ground stations, designed to be used with the ACTS. The purpose was to demonstrate the feasibility of a space-based high data-rate communications network. GSN provided wideband services and extensions of terrestrial fiber networks through ACTS. The interfaces and service conformed to SONET (Synchronous Optical Network) standards. Each ground station was capable of transmitting and receiving data at rates up to 622 Mbit/s, compatible with terrestrial SONET OC-3 and OC-12 service. Although only five ground stations were constructed, the system could accommodate up to 64. The network could be configured to operate with the fixed beams, scan beams and the steerable antenna of ACTS. 33)

• Mobile experiments using ACTS: NASA/JPL developed an experimental AMT (ACTS Mobile Terminal) where a fixed station (hub) communicates through the satellite with the mobile terminal. In the FDMA (Frequency Division Multiple Access) scheme, an unmodulated pilot signal is transmitted from the fixed station to the mobile terminal user through ACTS. The pilot is used by the mobile terminal to aid in antenna tracking, and as a frequency reference for Doppler offset correction and pre-compensation. For system efficiency, the pilot signal is only transmitted in the forward direction (fixed station-to-ACTS-to-mobile terminal. Operational data rates for this mobile terminal range from 2.4, 4.8, 9..6, 64, to 128 kbit/s.
The ACTS mobile experiments have demonstrated to use of Ka-band to provide voice and data communications to mobile terrestrial and aeronautical users. Both reflector and phased array antennas have been employed and techniques have been developed to maintain contact with the spacecraft as the mobile vehicle changes orientation and position along its travel. Data has also been collected to show the effect of obstacles along the path of travel for terrestrial vehicles. 34)

Some background of the ACTS program:

In 1978, as a result of the Presidential Directive, NASA began the process of rebuilding its R&D activities in the communication satellite arena. The future technology program was planned in cooperation with the National Research Council’s Space Applications Board Subcommittee on Satellite Communications, whose membership consisted of leading common carriers, spacecraft manufacturers, and representatives of communication users. 35)

Early communication satellite systems employed simple, bent-pipe transponders with a single antenna beam to cover a large region (such as the continental United States). The new NASA program needed to develop technology that would allow the frequency spectrum to be used more efficiently. One technique to accomplish this was to cover the region with many small spot beams so that the same frequency could be reused simultaneously in non-adjacent beams. Such frequency reuse increased the capacity of satellites by a factor of five to ten times that of a single beam satellite, with only a modest increase in spacecraft size, power, and weight. The technology to accomplish this high degree of frequency reuse employed antennas with high-gain spot beams and electronic systems with onboard switching and processing to inter-connect the spot beams. In addition, the high-gain antenna allowed for smaller aperture user terminals at higher data rates. This was the technology developed by NASA.

Technology Feasibility & Flight System Definition: In 1980, the ACTS program moved forward in two phases. The first phase was to 1) continue the market studies to increase confidence in the forecast for orbit saturation and 2) to do proof-of-concept development of the identified technologies. The proof-of-concept program was a laboratory (breadboard) type of development to prove that the technologies were feasible. Approximately $50 million was expended on the first phase. If the first phase proved successful, the second phase would consist of developing an experimental flight system to demonstrate that the technologies could provide reliable communications services.

The first phase was fully supported by the entire service provider and satellite manufacturing community. The second phase of the program was the one that became controversial. The service providers had great concern about how reliably the technology would work in space, and therefore, argued for a flight program. Some satellite manufacturers, however, had reservations about proceeding with a flight program because they felt it would give the winning contractors of the NASA procurement an unfair competitive advantage. This controversy continued throughout most of the life of the ACTS program.

Program Coordination with Industry: Two industry committees were formed to guide the program. The NASA Ad Hoc Advisory Committee was created to provide general policy direction. The committee included notable representatives of both the system supplier and service supplier industry. Their contribution provided timely and sage review of the program, as well as providing NASA with insight into the industry philosophy relative to the roles and responsibilities of both government and the private sector.

The second industry committee was a Carrier Working Group (CWG), consisting of representatives from all the major satellite service providers. The CWG was charged with helping NASA formulate the technology and flight system requirements, develop experiments, and provide overall guidance. These requirements and experiments were deemed necessary by the CWG to demonstrate the readiness of not only the technology, but of its service applications as well. Coordination was also established between the Department of Defense and NASA, especially in the development of various critical advanced technology components.

Proof-of-Concept Development: The purpose of the proof-of-concept (POC) technology development was to demonstrate the technical feasibility of the key component building blocks. The approach NASA used was to issue multiple contracts to various aerospace and related companies for the development of each high risk technology: multiple spot beam antenna, baseband processor, TWTA, wide-band switch matrices, low-noise receiver, GaAs FET transmitter, GaAs IMPATT (Impact Transit Time) transmitter, and ground antenna. Duplicate awards for most of the critical technology components were employed to increase the probability of successful development, and to produce multiple sources for communication hardware. In addition, multiple awards helped to ensure that a variety of perspectives and technical approaches were brought into each development. These contracts called for the development of the technology, the construction of POC versions of the components, and their testing in the laboratory to verify performance.

The POC hardware substantially reduced the risk associated with the planned development of the flight system. Another product of these technology contracts was the prediction of feasible component, subsystem, and system performance levels. NASA used these performance predictions to provide guidance for follow on technology development. Service providers and manufacturers could also use these predictions in planning activities for the commercial system designs of the early 1990s.

The Department of Defense (DoD) participated in the NASA POC program. Several of the critical technology POC elements that were of interest to the DoD were co-funded by DoD and NASA. To enable the effective transfer of information that was generated in the program, all contractors were required to prepare task completion reports. These reports were presented at periodic industry briefings (only for interested U.S. parties) hosted by NASA.

The NASA ACTS program served a very important role in advancing satellite communication because the commercial satellite communication industry in the 1980s could not afford to take on the risk associated with the necessary technology.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.